It has been desired to reduce, to the least possible extent, the leak rate of a working fluid that leaks through gaps between stationary and rotating members of rotary machines, such as axial-flow turbines including steam turbines and gas turbines.
FIG. 8 shows a general steam turbine, i.e. a rotary machine. This steam turbine has a turbine rotor 1 provided with rotor blades 2, i.e. rotating members, and a turbine casing 6 enclosing outer rings 3 of a nozzle diaphragm, stator blades (turbine nozzles) 4, and inner rings 5 of a nozzle diaphragm, i.e. stationary members. Opposite ends of the rotor 1 are supported in bearings 11. Noncontact sealing devices are interposed between each of the rotor blades 2 and each of the outer rings 3, and between the rotor 1 and each of the inner rings 5 to prevent the leakage of the working fluid.
FIG. 9 is an enlarged view of a part of FIG. 8 showing such a sealing device by way of example. Referring to FIG. 9, the outer ring 3 of the nozzle diaphragm holds a labyrinth packing 9 provided with a plurality of chip fins 7 extending toward the outer edge of the rotor blade 2. The inner ring 5 of the nozzle diaphragm 5 holds a labyrinth packing 10 provided with a plurality of sealing fins 8 extending toward the rotor 1.
The so-called hi-lo labyrinth sealing device, which is different from the sealing device shown in FIG. 9, has a protrusion disposed on one side, i.e. the rotating side or the stationary side, of a gap, and a plurality of sealing fins disposed on the other side of the gap. The sealing fins are arranged axially at equal pitches. The so-called comb-type labyrinth sealing device shown in FIG. 10 is used. The comb-type labyrinth sealing device shown in FIG. 10 has a plurality of sealing fins F1 and F2 disposed on the opposite sides of a gap, respectively, so as to extend toward each other. The fins F1 are arranged axially at equal pitches P2, and the fins F2 are arranged axially at equal pitches P1.
A rotary machine using a high-temperature working fluid, such as a steam turbine, undergoes temperature changes between starting and stopping and, consequently, the casing and the rotor of the rotary machine are displaced axially relative to each other due to the difference in thermal expansion, i.e. the difference in elongation, between the casing and the rotor. Generally, the rotor and the casing have different heat capacities, respectively, and the axial elongation of the casing is smaller than that of the rotor. Consequently, the rotor and the casing are displaced axially relative to each other.
In the comb-type sealing device as shown in FIG. 10, the sealing fins F1 and F2 do not come into contact with each other even if the rotor and the casing are displaced axially relative to each other due to the difference in thermal expansion between the rotor and the casing. On the other hand, the sealing performance of this comb-type sealing device is very high while the clearances between the corresponding sealing fins F1 and F2 are kept at a minimum. When the sealing fins F1 and F2 are displaced axially relative each other due to the difference in thermal expansion between the rotor and the casing, the clearances increase and the sealing performance of the comb-type sealing device deteriorates sharply.
The deterioration of the sealing performance due to the difference between the rotor and the casing in thermal expansion may be compensated for by axially arranging many sealing fins. However, the possible number of stages of the sealing fins is limited because the blowby of the fluid flowing in a sealing part occurs if the number of the stages of the fins is greater than a certain limit.
The sealing effect of the sealing device shown in FIG. 9 is higher when the clearances between the edges of the tip fins 7 and the sealing fins 8, and the rotating members 2 and 1 are smaller. However, it is possible that the edges of the fins 7 and 8 come into contact with the rotating members 1 and 2, and there by the fins 7 and 8, and the rotating members 1 and 2 are damaged if the clearances are excessively small.
The variation of the clearances due to thermal deformation during the operation of the rotary machine is the principal factor of such a trouble resulting from contact between the members. Such a variation of clearances occurs mostly at the start and stop of the rotary machine or when the load on the rotary machine changes. Therefore, if the size of the clearance is determined on the basis of conditions for operations other than those for the rated operation of the rotary machine, the clearances increase unnecessarily during the rated operation and, consequently, the leak rate of the working fluid increases. A movable sealing mechanism capable of changing clearances in a sealing part according to the operating condition is necessary to solve such a problem. A rotary machine provided with such a movable sealing mechanism is disclosed in, for example, JP61-16209A (1986). In addition, FIGS. 11 to 14 show known movable sealing mechanisms.
FIGS. 11 and 12 show the so-called hi-lo labyrinth sealing device including ridges 15 formed on the side surface of a rotor 1. A labyrinth packing 10 integrally provided with fins 13 is mounted on the inner ring 5 of a nozzle diaphragm. A proper radial clearance e is maintained between the fin 13 and the ridge 15 by the resilience of a plate spring 14 interposed between the labyrinth packing 10 and the inner ring 5 of the nozzle diaphragm.
A hi-lo labyrinth sealing device shown in FIGS. 13 and 14 includes a labyrinth packing 10 provided with circumferential grooves 16, and coil springs 17 placed in the circumferential grooves 16 of the labyrinth packing 10. In this hi-lo labyrinth sealing device, the radial clearance e between a fin 13 and a ridge 15 is regulated by the pressure of steam ST supplied through a groove 18 to the labyrinth packing 10, and the resilience of the springs 17 pressing the labyrinth packing 10 radially outward.
In those movable sealing mechanisms, the labyrinth packing 10 is moved radially with respect to the rotor 1. Thus, those movable sealing mechanisms are not provided with any measures for coping with the relative axial displacement of the casing and the rotor due to the difference between the casing and the rotor in thermal expansion (thermal elongation).
Therefore, in the rotary machines provided with those sealing mechanisms are obliged to arrange the plurality of ridges 15 axially at comparatively big pitches to avoid contact between the fins 13 and the ridges 15. The axial arrangement of the ridges at big pitches reduces working fluid sealing effect.